1. Field of the Invention
The present invention generally relates to polymeric membranes. Specifically, rigid polymeric membranes that go through a selectivity maximum as a function of copolymer composition and/or operating conditions, such as elevated temperature and/or feed pressure are described.
II. Brief Description of the Prior Art
The separation of one or more gases from a multicomponent mixture of gases is necessary in a large number of industries. Such separations currently are undertaken commercially by processes such as cryogenics, pressure swing adsorption, and membrane separations. In certain types of gas separations, membrane separations have been found to be economically more viable than other processes.
In a pressure-driven gas membrane separation process, one side of the gas separation membrane is contacted with a multicomponent gas mixture. Certain of the gases of the mixture permeate through the membrane faster than the other gases. Gas separation membranes thereby allow some gases to permeate through them while serving as a relative barrier to other gases. The relative gas permeation rate through the membrane is a property of the membrane material composition and its morphology.
It has been suggested in the prior art that the intrinsic permeability of a polymer membrane is a function of both gas diffusion through the membrane, controlled in part by the packing and molecular free volume of the material, and gas solubility within the material. Selectivity may be determined by the ratio of the permeabilities of two gases being separated by a material.
Transport of gases in polymers and molecular sieve materials occurs via a well known sorption-diffusion mechanism. The permeability coefficient (PA) of a particular gas is the flux (NA) normalized to the pressure difference across the membrane (xcex94pA), and the membrane thickness (l).                               P          A                =                              N            A                    ⁢                      l                          Δ              ⁢                              xe2x80x83                            ⁢                              p                A                                                                        (        1        )            
The permeability coefficient of a particular penetrant gas is also equal to the product of the diffusion coefficient (DA) and the solubility coefficient (SA).
PA=DASAxe2x80x83xe2x80x83(2) 
The permselectivity (xcex1A/B) of a membrane material (also ideal selectivity) is the ratio of the permeability coefficients of a penetrant pair for the case where the downstream pressure is negligible relative to the upstream feed pressure. Substituting equation (2), the ideal permselectivity is also a product of the diffusivity selectivity and solubility selectivity of the particular gas pair.                               α                      A            /            B                          =                                            P              A                                      P              B                                =                                                    D                A                                            D                B                                      ·                                          S                A                                            S                B                                                                        (        3        )            
The variation of gas permeability with pressure in glassy polymers is often represented by the dual mode model. Petropulos (1970); Vieth, et al. (1976); Koros, et al. (1977). The model accounts for the differences in gas transport properties in an idealized Henry""s law and Langmuir domains of a glassy polymer,                     P        =                                            k              D                        ⁢                          D              D                                +                                                    C                H                xe2x80x2                            ⁢                              D                H                            ⁢              b                                      1              +              bp                                                          (        4        )            
where kD is the Henry""s law constant, Cxe2x80x2H is the Langmuir capacity constant, p is pressure, and b is the Langmuir affinity constant. This model can be further extended to mixed gas permeability:                               P          A                =                                            k              DA                        ⁢                          D              DA                                +                                                    C                HA                xe2x80x2                            ⁢                              b                A                            ⁢                              D                HA                                                    1              +                                                b                  A                                ⁢                                  p                  A                                            +                                                b                  B                                ⁢                                  p                  B                                                                                        (        5        )            
where pA and pB are the partial pressures of gasses A and B respectively. This model is valid for a binary gas mixture of components A and B, and it only accounts for competitive sorption.
The temperature dependence of permeability for a given set of feed partial pressures is typically represented by an Arrhenius relationship:                     P        =                              P            o                    ⁢                      exp            ⁡                          [                                                -                                      E                    p                                                  RT                            ]                                                          (        6        )            
where Po is a pre-exponential factor, Ep is the apparent activation energy for permeation, T is the temperature of permeation in Kelvin, and R is the universal gas constant. The permeability can further be broken up into temperature dependent diffusion and sorption coefficients from equation (2). The temperature dependence of the penetrant diffusion coefficient can also be represented by an Arrhenius relationship:                     D        =                              D            o                    ⁢                      exp            ⁡                          [                                                -                                      E                    d                                                  RT                            ]                                                          (        7        )            
Again Do is a pre-exponential factor, and Ed is the activation energy for diffusion. The activation energy for diffusion represents the energy required for a penetrant to diffuse or xe2x80x9cjumpxe2x80x9d from one equilibrium site within the matrix to another equilibrium site. The activation energy is related to the size of the penetrant, the rigidity of the polymer chain, as well as polymeric chain packing. The temperature dependence of sorption in polymers may be described using a thermodynamic van""t Hoff expression:                     S        =                              S            o                    ⁢                      exp            ⁡                          [                                                -                                      H                    s                                                  RT                            ]                                                          (        8        )            
where So is a pre-exponential factor, and Hs is the apparent heat of sorption as it combines the temperature dependence of sorption in both the Henry""s law and Langmuir regions.
From transition state theory the pre-exponential for diffusion can be represented by                               D          o                =                  e          ⁢                      xe2x80x83                    ⁢                      λ            2                    ⁢                      kT            h                    ⁢                      exp            ⁡                          [                                                S                  d                                R                            ]                                                          (        9        )            
Here, Sd is the activation entropy, xcex is the diffusive jump length, k is Boltzmann""s constant, and h is Planck""s constant. Substituting (9) into (3) (neglecting small differences in the jump length of similarly sized penetrants) results in the diffusive selectivity as the product of energetic and entropic terms:                                           D            A                                D            B                          =                              exp            ⁡                          [                                                                    -                    Δ                                    ⁢                                      xe2x80x83                                    ⁢                                      E                                          d                      ,                      A                      ,                      B                                                                      RT                            ]                                ⁢                      exp            ⁡                          [                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                                      S                                          d                      ,                      A                      ,                      B                                                                      R                            ]                                                          (        10        )            
The diffusivity selectivity is determined by the ability of the polymer to discriminate between the penetrants on the basis of their sizes and shapes, and is governed primarily by intrasegmental motions and intersegmental packing. The diffusive selectivity will be based on both the difference in activation energy for both penetrants, xcex94Ed, as well as the difference in activation entropy for both penetrants, xcex94Sd.
Significant increases in diffusivity and diffusivity selectivity can be obtained in conventional polymers by simultaneously inhibiting intrasegmental motions and intersegmental chain packing. These results can be summarized as two principles for tailoring membrane materials:
1. Structural moieties which inhibit chain packing while simultaneously inhibiting torsional motion about flexible linkages on the polymer backbone tend to increase permeability while maintaining permselectivity;
2. Structural moieties which decrease the concentration of mobile linkages in the polymer backbone and do not significantly change intersegmental packing tend to increase permselectivity without decreasing permeability significantly.
The ratio of specific free volume to polymer specific volume, the fractional free volume, is representative of the degree of openness of the matrix. This index takes into account the filling of space by bulky side groups, but is not experimentally determined. The specific free volume is typically estimated by a group contribution method such as that of Bondi (1968) or Van Krevelen et al. (1976). The polymer specific volume is determined by dividing the molecular weight of the repeat unit by the bulk polymer density. The fractional free volume gives a measure of the degree of openness of the polymeric matrix. A relatively high fractional free volume is indicative of an open matrix, while a relatively low fractional free volume indicates a closed matrix. Materials with larger free fractional volumes are expected to have greater diffusivities (and sorption coefficients) and thus greater permeabilities than materials with smaller fractional free volumes.
Much of the work in the field has been directed to developing membranes that optimize the separation factor and total flux of a given system. It is disclosed in U.S. Pat. No. 4,717,394 to Hayes that aromatic polyimides containing the residue of alkylated aromatic diamines are useful in separating a variety of gases. Moreover, it has been reported in the literature that other polyimides, polycarbonates, polyurethanes, polysulfones and polyphenyleneoxides are useful for like purposes. U.S. Pat. No. 5,599,380 to Koros, herein incorporated by reference, discloses a polymeric membrane with a high entropic effect. U.S. Pat. No. 5,262,056 to Koros et al., herein incorporated by reference, discloses polyamide and polypyrrolone membranes for fluid separation.
U.S. Pat. No. 5,074,891 to Kohn et al. discloses certain polyimides with the residuum of a diaryl fluorine-containing diamine moiety as useful in separation processes involving, for example, H2, N2, CH4, CO, CO2, He and O2. By utilizing a more rigid repeat unit than a polyimide, however, even greater permeability and permselectivity are realized. One example of such a rigid repeat unit is a polypyrrolone.
Polypyrrolones as membrane materials were proposed and studied originally for the reverse osmosis purification of water by Scott et al. (1970). The syntheses, permeabilities, solubilities and diffusivities of polyimides has been described in (Walker and Koros (1991); Koros and Walker (1991); Kim et al. (1988a, b); Kim (1988c); Coleman (1992)). Membranes that are composed of the polyamide and polypyrrolone forms of hexafluoroisopropylidene-bisphthalic anhydride are disclosed in U.S. Pat. No. 5,262,056 which is incorporated herein by reference.
It is often desirable to perform separation processes under harsh conditions of high feed pressure and/or high temperature. However, typical polymeric membranes exhibit a decline in performance in these more aggressive environments. Conventional polymeric membranes, when subjected to high feed pressure and/or high temperatures, exhibit decreased selectivity. A need therefore exists for a polymeric membrane that improves separation performance under elevated temperature and pressure conditions. Furthermore, the ability to tune selectivity by altering the temperature and/or feed pressure would also be desirable. A membrane with these qualities would have a wide number of possible applications. For instance, such a polymer would be of particular use to the petrochemical industry.
In the petrochemical industry, one of the most important processes is the separation of olefin and paraffin gases. Olefin gases, particularly ethylene and propylene, are important chemical feed stocks. Various petrochemical streams contain olefins and other saturated hydrocarbons. These streams typically originate from a catalytic cracking unit. Currently, the separation of olefin and paraffin components is done using low temperature distillation. Distillation columns are normally around 300 feet tall and contain over 200 trays. This is extremely expensive and energy intensive due to the similar volatilities of the components.
It is estimated that 1.2xc3x971014 BTU per year are used for olefin/paraffin separations. This large capital expense and exorbitant energy cost have created incentive for extensive research in this area of separations. Membrane separations have been considered as an attractive alternative. Some examples of membranes that exhibit attractive selectivity under mild conditions have been reported. (Tanaka et al. (1996); Staudt-Bickel and Koros (2000); Ilinitch et al. (1993); Lee et al. (1992); Ito et al. (1989)). In practice, high propylene/propane temperatures and pressures are preferred for economical processing. Thus, a polymer membrane that showed enhanced propylene/propane selectivity under increasingly demanding processing conditions would be of particular value. Recent gas transport studies aimed at improving current membrane performance have examined glassy polymers focusing mainly on polyimides. Tanaka et al. (1996) have reported on the highest performance polyimides to date. This data has been used to construct a preliminary propane/propylene xe2x80x9cupper boundxe2x80x9d trade off curve between gas permeability and selectivity, as shown in FIG. 1. The conditions chosen for the upper bound curve are 2 atm feed pressure and 35xc2x0 C. The closed symbols in FIG. 1 represent pure gas polyimide data from the literature. The open symbols are pure gas data for other polymers from the literature (Tanaka et al. (1996); Staudt-Bickel and Koros (2000); Ilinitch et al. (1993); Lee et al. (1992); Ito et al. (1989); Steel (2000)). The propane/propylene upper bound trade off curve is poorly defined at this point in comparison to O2/N2 and CO2/CH4 xe2x80x9cupper boundxe2x80x9d curves (Robeson (1991)). It is believed that the membranes of the current invention provide performance beyond the upper bound for many gasses, including olefin/paraffin, O2/N2, and CO2/CH4 separations.
Described herein is a polymeric fluid separation membrane. In one embodiment the fluid separation membrane may be formed from the reaction product of a tetraamine, a tetraacid compound, and a diamine. The initial resulting product is a polyamide. This polyamide may be used to form a fluid separation membrane. Alternatively, the polyamide may be thermally cyclized to form a poly (pyrrolone-imide).
Fluid separation membranes formed from the herein described polyamides and poly (pyrrolone-imides) may exhibit unexpected properties when used under high temperature and/or pressure conditions. For example, when used at a relatively low first temperature and/or first pressure, the fluid separation membrane may exhibit low permeability, and low permselectivity. At an increased second temperature and/or second pressure, the fluid separation membrane may exhibit an increased permselectivity when compared to the permselectivity at the first temperature and/or pressure. The permselectivity of the fluid separation membrane may reach a maximum as the temperature and/or pressure is increased. If the temperature and/or pressure is increased to a third temperature and/or third pressure that are higher than the second temperature and/or pressure, the permselectivity may decrease.
The fluid separation membrane may be formed by adding a tetraacid compound to an amine mixture. The amine mixture may include tetramines and diamines. The tetraamine to diamine ratio may be between about 5:95 to about 100:0. After the tetraacid compound and the amines are reacted, the resulting polyamide may be filtered, washed and dried. The polyamide may be converted to a poly (pyrrolone-imide) by heating the polyamide to a temperature above about 200xc2x0 C. Either the polyamide or the polyimide may be used in as a fluid separation membrane.
The above-described fluid separation membranes may be used in any fluid separation apparatus known in the art. Generally, a fluid separation apparatus includes a body in which a fluid separation membrane is disposed. A fluid inlet may be positioned downstream from the fluid separation membrane. Two fluid outlets may be positioned upstream from the fluid inlet. A first fluid outlet may be positioned downstream from the fluid separation membrane. A second fluid separation membrane may be positioned upstream or downstream from the fluid separation membrane.
During use, a fluid stream that includes at least two components (e.g., a gas stream) may be introduced into the fluid separation apparatus via the fluid separation inlet. The fluid will then contact the fluid separation membrane. The fluid separation membrane may have a differential selectivity such that one of the components in the gas stream may pass through the fluid separation membrane at a rate that is faster than the rate at which the other component passes through. Thus the faster permeating component will pass through the gas separation membrane and flow out of the fluid separation apparatus via outlet. The gas that does not permeate through the membrane may exit the fluid separation apparatus via the outlet. The fluid stream passing out of the outlet may be recycled back into the fluid separation apparatus to improve the separation of the components and to maximize the yield of purifed components.